Pulse signals, such as pulse-width modulated (PWM) signals and pulse-duration modulated (PDM) signals, are used in controlling power applied to inertial devices and directly controlling average values, e.g. average flow-rate. A PWM or PDM signal is determined by two parameters: a PWM period and an on-time or duty cycle, which is the ratio of the on-time to the PWM period. Normally, in an application, the PWM period value in a PWM signal is a constant and is much lower than the time constant of the inertial device driven by the PWM signal, while in a PDM signal, the on-time value is fixed, and the period value is determined by the duty cycle value.
In addition to the parameters of pulse control signals, in a system with pulse control, there is a third factor, full-scale level, affecting control performance. The full-scale level is the control level during the on-time of a pulse signal. For example, in an electrical heater with PWM power control, the full-scale level is the electrical power delivered to the heater during the on-time of a PWM control signal, while in a system with a flow rate controlled by switching on and off a control valve with a PWM signal, the full-scale level is the flow rate when the control valve is switched on. The full-scale level together with duty cycle determines the average value controlled by a pulse signal.
In a feed-forward pulse control system, normally, a required duty cycle is generated based on a desired output value (target value), and then a pulse control signal is generated accordingly. If the full-scale level is constant and the calculation for duty cycle is accurate, then the control is accurate if there is no perturbation in the plant, i.e., there is no parameter change in the plant. However, due to variation in the full-scale level and perturbation in the plant, the actual output value may not equal to the target value. For example, in an electrical heater-temperature PWM control, if the driving voltage is not constant, then the actual power output varies even the duty-cycle of the PWM control is controlled constant and there is no change in heater resistance. In a flow-rate control, if the driving pressure changes, then the actual flow rate changes with the same valve open time even there is no change in other parameters such as nozzle shape and size.
A feedback control, which compares an output sensing value to a target value, and then adjusts the pulse signal accordingly, can be used to decrease the effects of variation in the full-scale level and other plant parameter changes. However, normally the feedback controller is designed for a nominal plant assuming constant plant parameter values. As a result, the variation in the full-scale level, and other plant parameter changes are perturbations to the control system. If the perturbations are much slower than the plant dynamics and the variation of plant parameters is small, then normally the feedback controller is able to correct the error caused by the variation and effectively decrease its effects. However, if the variation is large, then there would be a significant perturbation in the control system, while if the variation is fast, even it is small, it is a noise to the control system. Though for the perturbations, increasing the robustness of the feedback controller helps improving the stability of the system, and narrowing the band-width of the feedback controller de-sensitizes the system to the noise, all these methods lead to deterioration of the control performance.
In systems requiring accurate controls, variations in plant parameters need to be controlled small. However, to keep the variations small, in addition to decreasing plant sensitivity to influencing factors, a high precision driving source is also needed. For example, in an electrical heater-temperature PWM control system, a high precision voltage supply and a heater with low temperature coefficient are required for accurate temperature control, and in a flow-rate control system, a high precision driving pressure is needed. The high precision driving source and robust plant insensitive to influencing factors increase system cost and sometimes are not available. To solve this problem, a primary object of the present invention is to provide a control means to compensate the variation in the full-scale level and other plant parameters in the control loop and thereby eliminate its effects.
A further object is to include the control means into the pulse signal generation, so that the compensation to the variation in the full-scale level and other plant parameters won't increase the complexity of the system control, which provides commands to the pulse signal generation.
A further object is to generate a high precision pulse signal the precision of which is independent to the compensation of the control means added into the pulse generation.